Field of the Invention
The present invention generally relates to a three-dimensional-shape measurement apparatus, a three-dimensional-shape measurement method, and a non-transitory computer-readable storage medium, particular to a technique for measuring the three-dimensional shape of a measurement target object.
Description of the Related Art
There is widely known a three-dimensional measurement apparatus that obtains three-dimensional coordinates by the triangulation principle based on a position where an image capturing unit observes reflection light when a projection unit such as a projector projects a striped pattern typified by a space encoding method or the like onto a measurement target. With such an apparatus, the measurement accuracy of three-dimensional coordinates largely depends on the material of a measurement target.
In general, as for a measurement target made of a material such as a plastic or the like, the measurement accuracy deteriorates or the measurement itself becomes impossible due to a phenomenon called subsurface scattering or internal scattering. At the time of measurement of such a target, processing of the target, for example, by applying in advance a white powder or the like to the surface of the target is required, thereby considerably limiting the application possibilities of the three-dimensional measurement apparatus.
Patent literature 1 (Japanese Patent Laid-Open No. 2008-281399) discloses a method of accurately measuring the three-dimensional shape of a translucent object to be measured by providing a linearly polarizing plate on an optical path to separate surface-reflection light and internal scattered light.
Patent literature 2 (Japanese Patent Laid-Open No. 2012-251893) discloses a method of estimating the reflection-position coordinates of an object by projecting, in advance, a pattern onto a measurement target to acquire a reference luminance pattern, and comparing a luminance pattern observed at the time of measurement with the reference luminance pattern. This method is not limited to a translucent object, and it is possible to perform three-dimensional shape measurement at high accuracy regardless of the reflection characteristic of an object.
Non-patent literature 1 (Tongbo Chen, Hans-Peter Seidel, Hendrik P. A. Lensch, Modulated phase-shifting for 3D scanning, CVPR 2008) proposes a three-dimensional-shape measurement method which is hardly influenced by internal scattering by modulating a sinusoidal pattern in a phase shift method by a high-frequency, sinusoidal pattern.
Non-patent literature 2 (Tatsuhiko Furuse, Shinsaku Hiura, and Kosuke Sato, “3-D Shape Measurement Method with Modulated Slit Light Robust for Interreflection and Subsurface Scattering”, MIRU2009, Meeting on Image Recognition and Understanding) proposes a three-dimensional-shape measurement method which is hardly influenced by internal scattering by modulating slit light by an M-sequence including high frequency components.
In the method described in patent literature 1, however, the geometric relationship among a light source, light-receiving unit, and a measurement target surface needs to satisfy a given condition to separate surface-reflection light and internally-scattered light using polarization. Performing the measurement so as to satisfy the condition can be difficult.
In the method described in patent literature 2, the waveform of the luminance pattern depends on a number of parameters such as the shape of the target object and the geometric relationship between a light source and a light-receiving unit, in addition to the material of the measurement target object. To measure a wide range of objects, therefore, it is necessary to acquire, in advance, an enormous number of reference luminance patterns to cover many combinations.
Similarly, in the methods described in non-patent literatures 1 and 2, it is necessary to perform shooting by projecting a number of patterned light beams in order to measure the overall target object, resulting in a long measurement time.